The present invention relates to the generation of a halftone signal by comparison between an original gray scale value and a threshold value having a prescribed ordered-dither distribution.
Facsimile receivers and dot printers are of the type wherein the printing of dots is essentially an all or nothing binary process. While this all or nothing process is desirable for printing alphanumeric characters or other symbols, it is disadvantageous for printing images having gray scales or light gradations in continuous tones. This problem has been solved by transforming the continuous tone of the original image into halftone or pseudo halftone images using a mesh distribution or ordered-dither distribution known as "dither". Pseudo halftone images comprise a varying number of dots corresponding to the shades or tones to be reproduced. When the dots are small compared to the visual sharpness of the human eye, they are subliminal to the eye and are not recognized. Thus, dots and spaces visually fuse together on a writing surface to present a continuous tone.
One typical example of pseudo halftone generators as shown and described in U.S. Pat. No. 4,032,978 comprises a pseudo-random number generator and a comparator that compares input gray scale levels Xij (where i and j are representative of the row and column positions of a matrix array, respectively, in which dots and spaces are indicated) with the output of the pseudo-random number generator to produce a print signal Gij depending on whether the gray scales are higher or lower than a "dithered" reference Dij represented by the pseudo-random number. The reference Dij is varied according to a prescribed pseudo-randomness, or probability of occurrences in the directions of the rows and columns so that the output Gij has a distribution in which the ratio of dots and spaces varies with the prescribed randomness.
It is desirable to provide different distributions of dots and spaces according to different shades or optical density to improve continuous tones. However, the prior art halftone generator is not satisfactory in terms of the degree of freedom for selecting a desired dither distribution of threshold values.